1. Field of the Invention
This invention relates to a wavelength converter suitable for use for wavelength-division-multiplexing (WDM) optical communication.
2. Description of the Related Art
In recent years, WDM optical communication has been put into practical use as a large-capacity optical communication system. In WDM optical communication, a plurality of optical signals of different wavelengths are multiplexed, and communication channels are allocated individually to the multiplexed wavelengths. For WDM optical communication in the future, implementation of an optical switching function for switching information between arbitrary communication channels is being investigated, and various wavelength converters such as those described below have been proposed.
For example, Tajima et al. have proposed a symmetric Mach-Zehnder type all-optical switch including a Mach-Zehnder type interferometer (Japanese Patent Laid-Open No. 20510/1995 or Japanese Journal of Applied Physics, Vol. 32, pp.L1746-L1749, 1993). While the symmetric Mach-Zehnder type all-optical switch disclosed in the documents has been proposed for design a demultiplexer, since an output pulse of wavelength light different from that of an input pulse can be obtained (Nakamura et al., Applied Physics Letters, Vol. 65, pp.283-285, 1994), it can be used also as a wavelength converter (hereinafter referred to as first prior art). Tajima et al. have further proposed a polarization-discriminating type all-optical switch which is an improved apparatus of the first prior art and is higher in stability (Applied Physics Letters, Vol. 67, pp.3709-2711, 1995).
Meanwhile, Patel et al. have reported a polarization-discriminating type all-optical switch which operates with the same mechanism as the switch described above (IEEE Photonics Technology Letters, Vol. 8, pp.1695-1697, 1996). Also this polarization-discriminating type all-optical switch (hereinafter referred to as second prior art) can be used as a wavelength converter similarly to the first prior art.
Further, Ueno et al. have proposed a wavelength converter (hereinafter referred to as third prior art) which has a structure simplified from that of the second prior art (IEEE Photonics Technology Letters, Vol. 10, pp.346-348, 1998, The extended abstracts of the 58th autumn meeting of the Japan Society of Applied Physics, No. 3, p.1138, October, 1997, 5a-ZB-6, and The extended abstracts of the 45th spring meeting of the Japan Society of Applied Physics and Related Societies, No. 3, p.1135, March, 1998, 29a-SZL-17).
The first to third prior arts described above are wavelength converters which convert the wavelength of a Return-to-Zero (RZ) optical signal and can output an optical pulse shorter than the carrier lifetime in a nonlinear semiconductor waveguide).
Duurhaas et al. have proposed a wavelength converter which operates with a mechanism different from that of the first to third prior arts described above (Journal of Lightwave Technology, Vol. 14, pp.942-954, 1996, hereinafter referred to as fourth prior art). The fourth prior art is a wavelength converter which converts the wavelength of a Non-Return-to-Zero (NRZ) optical signal.
However, since the wavelength converters of the first to fourth prior arts described above make use of variation in refractive index of a semiconductor to convert the wavelength, an output optical signal suffers from wavelength shifting or wavelength chirping. Particularly an output signal of the fourth prior art exhibits intense wavelength chirping.
An optical signal with which wavelength chirping occurs is deteriorated in transmission characteristic because the spectrum width thereof is broadened when compared with another optical signal with which no wavelength chirping occurs. For example, if the wavelength converter of the fourth prior art is used for WDM optical communication, then this gives rise to such a problem that the cross talk between communication channels increases and the transmission characteristic relies upon the sign of a group velocity dispersion of the transmission line.
In order to narrow the spectrum width of an optical signal, a wavelength filter should be used. This, however, gives rise to another problem that the S/N ratio is deteriorated or an optical pulse is distorted in shape.
On the other hand, with regard to the first to third prior arts, problems such as occurrence of significant wavelength chirping and deterioration in wavelength conversion efficiency are not apparent because investigation or research of wavelength shifting or wavelength chirping of an optical signal has not been performed till now.